Light-emission control circuit

ABSTRACT

A light-emission control circuit may include n light-emitting devices, where n is an integer equal to or greater than 2; a light guide plate which causes light emitted by the n light-emitting devices to reach the entirety of the back of a liquid crystal panel; a measuring unit for measuring a luminance of the entirety of the n light-emitting devices operating in the liquid crystal panel; and a control unit for comparing the measured luminance with a predetermined value and controlling the luminance of the n light-emitting devices, based on a result of comparison, wherein the n light-emitting devices are attached to a side of the light guide plate, and the measuring unit is attached to an opposite side of the light guide plate.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 10/890,459 filed Jul. 12, 2004, the entire contentsof which are incorporated herein by reference and priority to which isclaimed herein. The 10/890,459 application claimed benefit of the dateof the earlier filed Japanese Patent Application No. 2003-322294 filedSep. 12, 2003 which is incorporated herein by reference, and priority towhich is claimed herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a light emission controlcircuits and, more particularly, to a circuit for controlling lightemission of light-emitting devices for illumination of a liquid crystalpanel.

2. Description of the Related Art

Liquid crystal display apparatuses are widely used in portable terminalssuch as a notebook personal computer. A liquid crystal panel in a liquidcrystal display apparatus is illuminated from behind by a backlight sothat desired display is obtained as a result of blocking andtransmission of light caused by interaction between a liquid crystal anda polarizing filter. A liquid crystal display apparatus is generallyknown for its low power consumption and will continue to be one of thekey display apparatuses.

A line light source such as a fluorescent tube, which is usually capableof illuminating the entirety of a panel at an equal level, is used in apersonal computer due to the availability of a substantially sufficientamount of space for a light source. In a portable terminal, however,there is a need to avoid a light source that requires an excessivelylarge housing space or an extremely high lighting voltage. For thisreason, many portable terminals today use light-emitting diodes capableof being lighted at a relatively low voltage. Japanese Patent Laid OpenPublication 2000-180850 proposes a structure of a liquid crystal displayapparatus using the diode.

An idea should be introduced in order to allow the light emitted bylight-emitting diodes to reach the entirety of a panel at an equallevel. Japanese Patent Laid Open Publication mentioned above disclosesreduction of ununiformity of light using a light guide panel but doesnot, however, discloses how the degree of ununiformity is managed nordoes it disclose taking into account unit-to-unit variation ortime-dependent change of a light source. Granting that the light iscontrolled to be uniform, the disclosure in the patent documentoverlooks too high or too low an overall luminance level. A combinationof red (R) green (G) and blue (B) light-emitting diodes to providebacklight is not capable of desirable light emission unless a properluminance balance is achieved. The patent document is also silent as tohow the luminance balance is managed.

Related Art List

JPA laid open 2000-180850

SUMMARY OF THE INVENTION

The present invention is done in the existing state of technologydescribed above and has an objective of providing a light-emissioncontrol circuit capable of providing a uniform backlight to a liquidcrystal panel using light-emitting devices or enabling desired backlightluminance and color tone.

The light-emission control circuit according to one aspect of thepresent invention comprises: n kinds of light-emitting devices eachproducing one of first through nth colors, where n is an integer equalto or greater than 2; a measuring unit for measuring the first throughnth color components included in a synthesized light produced by thetotal of n kinds of light-emitting devices operating in a liquid crystalpanel; and a control unit for adjusting the luminance of the total of nkinds of light-emitting devices, based on the measurement by themeasuring unit. With this construction, it is possible to adjust theluminance of light-emitting devices using feedback control. Since the nkinds of light-emitting devices produce different colors, an optimalcolor tone is produced by optimizing the luminance balance of thelight-emitting devices.

The light-emission control circuit according to another aspect of thepresent invention comprises: n kinds of light-emitting devices eachproducing one of first through nth colors, where n is an integer equalto or greater than 2; n measuring units each measuring one of the firstthrough nth color components included in a synthesized light produced bythe total of n kinds of light-emitting devices operating in a liquidcrystal panel; and a control unit for adjusting the luminance of thetotal of n kinds of light-emitting devices, based on the measurement bythe measuring units.

The light-emission control circuit according to still another aspect ofthe present invention comprises: n light-emitting devices, where n is aninteger equal to or greater than 2; a measuring unit for measuring theluminance of the total of n light-emitting devices operating in a liquidcrystal panel; and a control unit for adjusting the luminance of thetotal of n light-emitting devices, based on the measurement by themeasuring unit. According to this aspect of the invention, the nlight-emitting devices may produce the same color. In this case, desiredluminance is obtained using feedback control. The desired luminance maybe the overall luminance produced by the n light-emitting devices as awhole or the individual devices.

According to the light-emitting control circuit of the presentinvention, it is possible to provide a uniform light or a light ofdesired luminance or color tone to the entirety of a liquid crystalpanel.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and further features of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings, in which:

FIG. 1 schematically shows an overall construction in which alight-emission control circuit according to the first embodiment isapplied to a liquid crystal display apparatus.

FIG. 2 shows an overall construction of the circuit of FIG. 1.

FIG. 3 schematically shows an overall construction in which alight-emission control circuit according to the second embodiment isapplied to a liquid crystal display apparatus.

FIG. 4 schematically shows an overall construction in which alight-emission control circuit according to the third embodiment isapplied to a liquid crystal display apparatus.

FIG. 5 shows an arrangement in which a measuring unit and a light sourceare provided on the same surface of a light guide plate.

FIG. 6 shows an arrangement of light sources and light-receiving partsin the fifth embodiment.

FIG. 7 shows an arrangement in which a measuring unit is provided on aliquid crystal panel.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments according to the invention will be described briefly.The light-emission control circuit according to the first embodiment andthe sixth embodiment, a variation of the first embodiment, comprisesthree light-emitting diodes producing R, G and B colors, respectively;three measuring units each measuring one of the R, G and B colorcomponents included in a synthesized light produced by the threelight-emitting diodes operating in a liquid crystal panel; and a controlunit for adjusting the luminance of the light-emitting diodes, based onthe measurement by the measuring units. As another embodiment relevantto the measuring unit, the light-emission control circuit may comprise ameasuring unit for measuring the R, G and B color components. Themeasuring units are implemented, for example, by photodetectors such asphotodiodes and phototransistors. In this construction, the luminance ofthe three kinds of light-emitting diodes are adjusted so that the lightbefore transmission through the liquid crystal panel matches a referencewhite light (first embodiment) or the light after transmission throughthe liquid crystal panel matches a reference white light (sixthembodiment). Therefore, the first and sixth embodiments are designed toadjust the balance of luminance produced by the plurality oflight-emitting devices and focuses on the color tone of the synthesizedlight. Although the R, G and B colors were discussed above, therequirement is that the luminance of light-emitting devices such aslight-emitting diodes corresponding to a plurality of colors isadjusted. The number of the light-emitting devices provided for eachcolor is irrelevant to the purpose of the invention.

The second through fifth embodiments focus on the overall luminanceproduced by a plurality of light-emitting diodes as a whole or theluminance of the individual light-emitting diodes. The light-emissioncontrol circuit according to the second embodiment comprises: aplurality of light-emitting diodes; a measuring unit for measuring theluminance of the light-emitting diodes operating in a liquid crystalpanel; and a control unit for adjusting the luminance of thelight-emitting diodes, based on the measurement by the measuring unit.Unlike the first and sixth embodiments, the plurality of light-emittingdiodes may produce the same color. Hereinafter, it is assumed that theplurality of diodes produce a white light. The produced color may be thesame since the second through fifth embodiments only focus on theluminance. The control unit may effect control such that the pluralityof light-emitting diodes produce the same luminance (fifth embodiment)or the overall luminance produced by the diodes as a whole is comparedwith a predetermined value and controlled collectively, based on themeasurement result (second through fourth embodiments).

According to the first and sixth embodiments that focus on color tone,it is possible to match the color tone of a backlight to a referencecolor and cancel unit-to-unit variation of light-emitting diodes ortime-dependent change in characteristics. According to the secondthrough fifth embodiments focusing on luminance, it is possible to matchthe overall luminance of a backlight to a reference value or provide auniform illumination in which nonuniformity in the luminance of abacklight is eliminated. In a similar configuration as the first andsixth embodiments, the second through fifth embodiments is capable ofcanceling unit-to-unit variation etc. of the light-emitting diodes.

FIRST EMBODIMENT

FIG. 1 schematically shows an overall construction in which thelight-emission control circuit according to the first embodiment isapplied to a liquid crystal display apparatus. The liquid crystaldisplay apparatus is provided with a liquid crystal panel 10 and abacklight unit 12. The liquid crystal panel 10 may be any transmissiveor semi-transmissive liquid crystal panel. For example, a twist nematic(TN) liquid crystal, a super twist nematic (STN) liquid crystal or athin-film transistor (TFT) liquid crystal may be used. The constructionof the liquid crystal panel 10 is not illustrated in detail since it ispopularly known. For example, a polarizing filter, a glass or plasticsubstrate, a transparent electrode, an alignment layer, a liquid crystalmaterial, an alignment layer, a transparent electrode, a color filter, aglass substrate, a polarizing plate are arranged in the ascending orderof distance from the backlight unit 12. The liquid crystal materialfills a space between the alignment layers. The liquid crystal panel 10is integrally formed.

The backlight unit 12 is also integrally formed such that a light guideplate 18 that causes the light emitted by the light source and enteringthe side of the plate to reach the entirety of the back of the liquidcrystal panel 10, a diffuser plate 16 provided on the surface of thelight guide plate 18 and a color filter 14 transmitting the diffusedlight are provided in the descending order of distance from the liquidcrystal panel 10. The color filter 14 is formed by polyethyleneterephthalate (PET) or polycarbonate (PC) resin. The diffuser plate 16is formed by roughening the surface of a transparent resin plate. Thediffuser plate 16 should be made of a material characterized by a smalllight loss and a high light diffusion coefficient. Polycarbonate (PC)resin or acrylic resin, characterized by a relatively high transparency,is used to form the light guide plate 18. A light-reflecting mechanism(not shown) is provided in a housing to face the back of the light guideplate 18. In an apparatus in which the housing is not provided with thereflecting mechanism, a reflecting plate (not shown) is adhesivelyattached to the back of the light guide plate 18. FIG. 1 contains anexploded view for ease of understanding of the construction and thescale thereof may be different from the actual scale.

Adhesively attached to the side of the light guide plate 18 is a firstlight-emitting part 21, a second light-emitting part 22 and a thirdlight-emitting part 23. Each of these parts emits a white light.Independent R, G and B light-emitting diodes are provided in each of thelight-emitting parts. The opposite side of the light guide panel 18 isprovided with a first light-receiving part 31, a second light-receivingpart 32 and a third light-receiving part 33. The light-receiving partsconstitute a measuring unit. The first light-receiving part 31, thesecond light-receiving part 32 and the third light-receiving part 33detect R, G and B color components, respectively. The result ofdetection is sent to a control unit 40.

The control unit 40 refers to the result of detection by the threelight-receiving parts and checks the RGB luminance balance against areference level. The control unit 40 controls how the firstlight-receiving part 31, the second light-receiving part 32 and thethird light-receiving part 33 are driven in accordance with theluminance level. As a result of feedback control, a desired tone ofcolor is produced.

FIG. 2 shows an overall construction of the circuit of FIG. 1. Each ofthe first light-emitting part 21, the second light-emitting part 22 andthe third light-emitting part 23 at the center of the figure includes R,G and B light-emitting diodes. The three R light-emitting diodesincluded in the first light-emitting part 21, the second light-emittingpart 22 and the third light-emitting part 23 are coupled in seriesbetween a power supply and a first constant-current supply 60.Similarly, three G light-emitting diodes are coupled in series betweenthe power supply and a second constant-current source 62, and three Blight-emitting diodes are coupled in series between the power supply anda constant-current supply 64. A white light resulting from thecombination of R, G and B components and emitted by the firstlight-emitting part 21, the second light-emitting part 22 and the thirdlight-emitting part 23 is led to the light guide plate 18. A synthesisof the incident white light reaches the measuring unit via the lightguide plate 18.

The measuring unit is provided with the first light-receiving part 31,the second light-receiving part 32 and the third light-receiving part33. These parts measure the luminance of R, G and B light components,respectively. The first light-receiving part 31 is provided with a firstphototransistor 31 a and a first resistor 31 b coupled in series betweena power supply and the ground. A result of detection is output as avoltage signal from a node between the transistor and the ground. A red(R) filter 31 c is provided between the first phototransistor 31 a andthe light guide plate 18 so that the phototransistor 31 a detects onlythe R component. The second light-receiving part 32 and the thirdlight-receiving part 33 are constructed similarly. The filters aredenoted by respective reference symbols in the figure, a green (G)filter 32 c provided in the second light-receiving part 32 beingtransparent only to the G component and a blue (B) filter 33 c providedin the third light-receiving part 33 being transparent only to the Bcomponent. For prevention of entry of ambient light, the spacesandwiched by the light guide plate 18, the first light-receiving part31, the second light-receiving part 32 and the third light-receivingpart 33 is masked or similarly processed.

The control unit 40 is provided with a first A/D converter 51, a secondA/D converter 52 and a third A/D converter 53, the A/D converter beingdenoted in the figure by a reference symbol “ADC”. The A/D convertersreceive the measurement result from the first light-receiving part 31,the second light-receiving part 32 and the third light-receiving part 33in the form of a voltage signal. The converters convert the signal intoa digital value and output the same to a microprocessor 54. The digitalvalue derived from the measurement of R, G and B components will bereferred to as an R measurement, a G measurement and a B measurement,respectively. These three measurements will generically be referred toas measurements.

A memory 58 has a ROM and a RAM built therein. The ROM is provided witha table containing an R reference value, a G reference value and a Breference value with which the R measurement, the G measurement and theB measurement are compared, respectively (the ROM, RAM and table are notshown). The R reference value, the G reference value and the B referencevalue are generically referred to as reference values. The referencevalues correspond to a target white light. User-specified color toneconfiguration information is written in the RAM of the memory 58 via anUI part 56 providing a user interface. The UI part 56 provides anenvironment necessary for operation for adjusting the displayed colortone according to the user's preference. When the user designates acolor tone, the RGB values (hereinafter, referred to as designatedvalues) corresponding to the designated color tone are given precedenceover the reference values.

The microprocessor 54 receives the measurements from the first A/Dconverter 51, the second A/D converter 52 and the third A/D converter53, and reads the designated value from the memory 58 so as to effectfeedback control for matching the measurements to the designated value.When the designated value is not provided, the microprocessor 54controls the measurements to match the reference values. Control iseffected independently in the first constant-current source 60, thesecond constant-current source 62 and the third constant-current source64. When the R component is insufficient, for example, themicroprocessor 54 drives the first constant-current source 60 so that itsupplies a larger current.

Thus, according to the first embodiment, it is possible to configure abacklight to produce a preset color tone or a color tone designated bythe user. The following variations of the first embodiment arepracticable.

1. The first constant-current source 60 may be a constant-voltagesource. In this case, if the R component is insufficient the voltageacross the three R light-emitting diodes may be raised so that thediodes emit light at a higher luminance.

2. A switch may be inserted in series with the first constant-currentsource 60 so that duty control such as pulse width modulation (PWM)control using the switch is effected. In this case, the light-emittingdiode is driven only when the switch is on so that desired luminance iseasily achieved by controlling the duty ratio.

3. Instead of subjecting the constant-current source to control, acombination of a constant-voltage source and PWM control may be used. Inthis case, desired luminance is easily achieved by controlling the dutyratio.

4. The control unit 40 according to the first embodiment usesalgorithmic control using the microprocessor 54. Alternatively, thecontrol may be effected using a circuit without the microprocessor 54.In this case, the control unit 40 may be replaced by three comparatorsystems. One of the terminals of the comparator may receive a voltagesignal from the light-receiving part such as the first light-receivingpart 31 and the other terminal may receive a reference voltage so thatthe output of the comparator may directly control the constant-currentsource such as the first constant-current source 60. Since themicroprocessor 54 is not necessary in this configuration, the processingload is reduced so that the processing speed is generally increased.

The variations described above are equally practicable in the followingembodiments.

SECOND EMBODIMENT

FIG. 3 schematically shows an overall construction in which alight-emission control circuit according to the second embodiment isapplied to a liquid crystal display apparatus. Hereinafter, thoseelements that are identical to the elements of FIG. 2 are designated bythe same reference symbols and the description thereof is omitted. Thefollowing description mainly concerns differences from the configurationof FIG. 2.

As shown in FIG. 3, according to the second embodiment, the R filter 31c, the G filter 32 c and the B filter 33 c are removed from the firstlight-receiving part 31, the second light-receiving part 32 and thethird light-receiving part 33 of the measuring unit, respectively. Thefirst phototransistor 31 a, the second phototransistor 32 a and thethird phototransistor 33 a directly detect light from the light guideplate 18. The microprocessor 54 controls the first constant-currentsource 60, the second constant-current source 62 and the thirdconstant-current source 64 collectively instead of individually. The ROMin the memory 58 contains a single reference value indicating theoverall luminance of the backlight. The RAM in the memory 58 contains auser-designated value relating to luminance. The microprocessor 54compares a simple sum or a linear sum of the measurements derived fromthe first light-receiving part 31, the second light-receiving part 32and the third light-receiving part 33 with the designated value or thereference value so as to effect feedback control. Whether the simple sumor the linear sum of the measurements should be used may be determinedby experiments or the like.

With the construction described above, the second embodiment makes itpossible to math the overall luminance of a backlight, instead of thecolor tone thereof, to the designated value or the reference value.

FIG. 3 shows the R light-emitting diodes of the first light-emittingpart 21, the second light-emitting part 22 and the third light-emittingpart 23 being connected in series, the G light-emitting diodes beingconnected in series and the B light-emitting diodes being connected inseries. A constant-current source is provided for each of the seriesconnection. Since the second embodiment is not designed for adjustmentof a color tone, the connection involving the constant-current sourceallows for extensive flexibility. For example, the three light-emittingdiodes in the first light-emitting part 21 may be connected in seriesand driven by the first constant-current source 60. Similarly, the threelight-emitting diodes in the second light-emitting part 22 and the thirdlight-emitting part 23 may be respectively connected in series so thatthe diodes are driven by the second constant-current source 62 and thethird constant-current source 64, respectively. Such a configuration, inwhich the first light-emitting part 21, the second light-emitting part22 and the third light-emitting part 23 are controlled independently,will be referred to an embodiment 2A. The microprocessor 54 according tothe embodiment 2A may control the first constant-current source 60, thesecond constant-current source 62 and the third constant-current source64 independently so that the measurements match the designated value orthe reference value, as well as controlling them collectively as in thesecond embodiment. For example, when the light detected by the firstlight-receiving part 31 is higher in intensity than the light detectedby the third light-receiving part 33, the third light-emitting part 23closer to the third light-receiving part 33 than the otherlight-emitting parts may be controlled to produce a higher amount oflight.

THIRD EMBODIMENT

FIG. 4 schematically shows an overall construction in which alight-emission control circuit according to the third embodiment isapplied to a liquid crystal display apparatus. Hereinafter, thoseelements that are identical to the elements of FIG. 3 are designated bythe same reference symbols and the description thereof is omitted. Thefollowing description mainly concerns the difference from theconfiguration of FIG. 3.

As shown in FIG. 4, according to the third embodiment, the firstlight-receiving part 31, the second light-receiving part 32 and thethird light-receiving part 33 of the measuring unit are replaced by thesingle light-receiving part 31. Correspondingly, the first A/D converter51, the second A/D converter 52 and the third A/D converter 53 arereplaced by the single A/D converter 51. The microprocessor 54 controlsthe first constant-current source 60, the second constant-current source62 and the third constant-current source 64 collectively. The ROM in thememory 58 contains a single reference value indicating the overallluminance of a backlight. The RAM in the memory 58 contains auser-designated value relating to the luminance. The microprocessor 54compares measurement by the first light-receiving part 31 with thereference value so as to effect feedback control.

With the construction described above, it is possible to match theoverall luminance of a backlight to the reference value, using a simplerconfiguration than the second embodiment. In a similar configuration asthe embodiment 2A, a variation to the third embodiment in which thefirst light-emitting part 21 etc. are independently controlled may bepracticable as an embodiment 3A.

FOURTH EMBODIMENT

In the third embodiment, the first light-receiving part 31 is providedon the light guide plate 18 opposite to the light source. In the fourthembodiment, these elements are provided in a coplanar arrangement. FIG.5 shows such an arrangement. The location of the first light-emittingpart 21, the second light-emitting part 22 and the third light-emittingpart 23 are the same as those of the third embodiment. The differencefrom the third embodiment is that the first light-receiving part 31 isprovided between the second light-emitting part 22 and the thirdlight-emitting part 23. The other aspects of the arrangement are thesame as the corresponding aspects of the third embodiment. By arrangingthe elements in close approximation to each other, designing of amechanism is made easier.

To monitor the overall luminance of a backlight, the light-receivingparts constituting the measuring unit may be placed in closeapproximation to a light source. No significant difference from thenon-coplanar arrangement exists since it is only necessary to performexperiments with the coplanar arrangement so as to know an appropriatereference value. The purpose is also served by placing thelight-receiving part at an angle of 90 degrees to the plane on which thelight source is provided. In principle, the light source and thelight-receiving part may be of any arrangement as long as the light fromthe former reaches the latter. In a similar configuration as theembodiment 2A, a variation to the fourth embodiment in which the firstlight-emitting part 21 etc. are independently controlled may bepracticable as an embodiment 4A.

FIFTH EMBODIMENT

In the fourth embodiment, only one light-receiving part is provided. Inthe fifth embodiment, the light-receiving parts are provided forrespective light sources so that the light sources may be controlledindividually. Thus, the fifth embodiment is based on a configuration inwhich the first light-emitting part 21 etc. are individually controlled,in a similar configuration as the embodiment 2A.

FIG. 6 shows an arrangement of the light sources and the light-receivingparts in the fifth embodiment. The first light-receiving part 31, thesecond light-receiving part 32, the third light-receiving part 33 and afourth light-receiving part 34 are provided in close approximation tothe first light-emitting part 21, the second light-emitting part 22, thethird light-emitting part 23 and a fourth light-emitting part 24,respectively. In this example, the liquid crystal panel 10 has alongitudinal symmetry so that the first light-emitting part 21, thesecond light-emitting part 22, the third light-emitting part 23 and thefourth light-emitting part 24 are symmetrically arranged with respect tothe center of the liquid crystal panel 10. The first light-receivingpart 31, the second light-receiving part 32, the third light-receivingpart 33 and the fourth light-receiving part 34 are similarly arranged.Theoretically, it is ideal in this construction that the firstlight-receiving part 31, the second light-receiving part 32, the thirdlight-receiving part 33 and the fourth light-receiving part 34 give thesame measurement. Of course, the first light-emitting part 21, thesecond light-emitting part 22, the third light-emitting part 23, thefourth light-emitting part 24, the first light-receiving part 31, thesecond light-receiving part 32, the third light-receiving part 33 andthe fourth light-receiving part 34 may not be arranged symmetrically. Inthis case, experimental measurements are taken from the first throughfourth light-receiving parts in a state in which the entirety of thebacklight unit 12 is illuminated uniformly and a desired overallluminance is achieved. The measurements are then set as the referencevalues. Thus, according to the fifth embodiment, it is possible toconfigure the luminance of individual light sources properly, inaddition to the overall luminance. As a result, ununiformity inluminance is reduced.

SIXTH EMBODIMENT

In the first embodiment, the first light-receiving part 31, the secondlight-receiving part 32 and the third light-receiving part 33 areadhesively attached to the side of the light guide plate 18 opposite tothe side on which the first light-emitting part 21, the secondlight-emitting part 22 and the third light-emitting part 23 areprovided. In the sixth embodiment, the first light-receiving part 31,the second light-receiving part 32 and the third light-receiving part 33are adhesively attached to the liquid crystal panel 10. FIG. 7 shows anarrangement involving the first light-receiving part 31, the secondlight-receiving part 32, the third light-receiving part 33 and theliquid crystal panel 10. The first light-receiving part 31, the secondlight-receiving part 32 and the third light-receiving part 33 areprovided on the upper surface of the liquid crystal panel 10 and withinan active display area. The first light-receiving part 31, the secondlight-receiving part 32 and the third light-receiving part 33 maypreferably be formed using low-profile components. Generally, the liquidcrystal panel 10 and the light guide plate 18 are integrally sealed in ahousing (not shown) and an opening in the housing for the liquid crystalpanel 10 is smaller than the liquid crystal panel 10. A certain marginalarea of the liquid crystal panel 10 is thus concealed by the housing.Accordingly, visual appearance does not suffer by arranging the firstlight-receiving part 31, the second light-receiving part 32 and thethird light-receiving part 33 at locations concealed by the housing.

With this construction, it is possible to detect the amount of lighttransmitted by the liquid crystal panel 10. Therefore, the user iscapable of knowing the color tone and the luminance as actuallyexperienced when viewing the liquid crystal panel 10 and matching thedetected color tone and luminance to the designated value or thereference value. It is also possible to effect control in whichtime-dependent change and unit-to-unit variation of the liquid crystalpanel 10 are taken into consideration.

In the sixth embodiment, the measurements depend on what is displayed onthe liquid crystal panel 10. Therefore, the microprocessor 54 maypreferably cause a known test image to be displayed at an initializingprocess at power-on so that the measurement and feedback control arecompleted while the image is displayed. The configuration according tothe sixth embodiment in which the light transmitted through the liquidcrystal panel 10 is detected is equally practicable in any of the secondthrough fifth embodiments as well as in the first embodiment.

The embodiments are described as examples and many other variations inthe constituting elements and the combination of individual processingunits are possible. It will be known to those skilled in the art thatthose variations are within the scope of the present invention. Somesuch variations will be discussed.

In the above-described embodiments, light-emitting diodes are used aslight-emitting devices. Alternatively, the light-emitting devices may beelectro-luminescence (EL) devices. Light-emitting devices producingdifferent colors or light-emitting devices producing an identical colormay be used as desired depending on the purpose of the embodiment.

In the above-described embodiments, the control unit 40 and themeasuring unit including the first light-receiving part 31 etc.constitute a light-emission control circuit. Alternatively, alight-emission control circuit may additionally comprise any one of aplurality of elements including a light source such as the firstlight-emitting part 21, the light guide plate 18, the backlight unit 12,the liquid crystal panel 10 and a driving circuit such as the firstconstant-current source 60.

1. A light-emission control circuit comprising: n light-emitting devices, where n is an integer equal to or greater than 2; a light guide plate which causes light emitted by the n light-emitting devices to reach the entirety of the back of a liquid crystal panel; a measuring unit for measuring a luminance of the entirety of the n light-emitting devices operating in the liquid crystal panel; and a control unit for comparing the measured luminance with a predetermined value and controlling the luminance of the n light-emitting devices, based on a result of comparison, wherein the n light-emitting devices are attached to a side of the light guide plate, and the measuring unit is attached to an opposite side of the light guide plate.
 2. A light-emission control circuit comprising: n light-emitting devices, where n is an integer equal to or greater than 2; a light guide plate which causes light emitted by the n light-emitting devices to reach the entirety of the back of a liquid crystal panel; a measuring unit for measuring a luminance of the entirety of the n light-emitting devices operating in the liquid crystal panel; and a control unit for comparing the measured luminance with a predetermined value and controlling the luminance of the n light-emitting devices, based on a result of comparison, wherein the n light-emitting devices and the measuring unit are adhesively attached to a same side of the light guide plate.
 3. A light-emission control circuit comprising: n light-emitting devices, where n is an integer equal to or greater than 2; a light guide plate which causes light emitted by the n light-emitting devices to reach the entirety of the back of a liquid crystal panel having longitudinal symmetry; a plurality of measuring units for measuring a luminance of the entirety of the n light-emitting devices operating in the liquid crystal panel; and a control unit for comparing the measured luminance with a predetermined value and controlling the luminance of the n light-emitting devices, based on a result of comparison, wherein the n light-emitting devices are symmetrically attached to a side of the light guide plate with respect to a center of the liquid crystal panel, and the plurality of measuring units are respectively attached in close approximation to the n light-emitting devices in one-to-one correspondence with n light-emitting elements.
 4. The light-emission control circuit according to claim 1, wherein the control unit comprises a memory for containing user-designated color tone configuration information and a microprocessor for controlling the luminance of the n light-emitting devices by feedback control so as to match measurements by the measuring unit to the color tone configuration information.
 5. The light-emission control circuit according to claim 2, wherein the control unit comprises a memory for containing user-designated color tone configuration information and a microprocessor for controlling the luminance of the n light-emitting devices by feedback control so as to match measurements by the measuring unit to the color tone configuration information.
 6. The light-emission control circuit according to claim 3, wherein the control unit comprises a memory for containing user-designated color tone configuration information and a microprocessor for controlling the luminance of the n light-emitting devices by feedback control so as to match measurements by the measuring unit to the color tone configuration information. 